This invention relates to a method and apparatus for radioactivity well logging and more particularly to techniques for producing radioactive logs indicative of the natural gamma radiation occurring in preselected energy bands.
As is well known in the art, natural gamma rays produced by the earth formations provide an indication of the occurrence of the naturally occurring radioactive elements, such as thorium, potassium and uranium. Since these elements produce gamma rays having different energy levels, a well logging instrument having the capability for measuring the energy of such gamma rays, for example, one having a scintillation counter, thus provides a means of ascertaining the existence of such elements.
Various methods and apparatus have been utilized in the well logging art to study such radioactive properties of subsurface formations. Accordingly, logs of such properties aid in the study of the nature of the subsurface formations, particularly in exploration for minerals and hydrocarbon deposits. It has been found that a correlation may be established between the uranium, potassium and thorium content of the subsurface formations which can be used as an indicator of the organic matter content therein. It is known that potassium, uranium and thorium are important natural sources of gamma radiation. Each of these elements either contains or radioactively decays to radioactive isotopes which emit gamma radiation at the above-mentioned characteristic energy levels. Further, the natural gamma rays spectrum for the given formation exhibits peaks of intensity at energies corresponding to the potassium, uranium and thorium content of the formation.
Unfortunately, gamma rays attributable to higher energy sources of radiation may decay to a lower energy level resulting in the contamination of the measurements of lower energy level sources. Thus, gamma rays having energy levels attributable to one element, for example, uranium may also include reduced energy level radiation caused by a higher energy level element such as thorium. Similarly, measurements taken of the radiation occurring in the energy level attributable to potassium may also include radiation counts attributable to uranium and thorium radiation.
As a result, the prior art has been directed to providing means of stripping lower energy level radiation of the radiation influence caused by the generation of the gamma radiation from higher energy band signals which have deteriorated. In one such prior art system, described in U.S. Pat. No. 3,940,610, issued Feb. 24, 1976 to Dennis et al, the measured gamma ray spectra are passed through three energy band selectors, each comprising a single channel discriminator and count rate meter to separate the output of the gamma ray detector into potassium, uranium and thorium energy band signals while at the same time converting the radioactive counts to analog voltages. These analog signals are then passed through operational amplifier circuits which have been calibrated to algebraically remove the influence resulting from higher energy level elements.
The use of analog stripping techniques, however, offers several disadvantages in operation and reliability. First, the counts are converted to voltage levels, with the accompanying inclusion of time constants. Thus, the data is partially processed prior to any stripping with a loss in data accuracy which could be obtained by stripping raw data.
Second, the difficulty in calibrating analog circuits reduces the repeatability of the measurements. Different logging runs through the same section of a borehole may give different results even when radiation levels remain constant. Further, in setting up the analog calibrations, time constants must be matched or the resultant measurement curves will have a different statistical appearance.
Third, the use of analog circuits permits the measurement curves to display values less than zero. This results in having to calibrate each analog circuit with respect to the other analog circuits with no means to branch when no measurements are available for one of the measured elements.
These and other disadvantages are overcome with the present invention wherein improved methods of stripping and filtering the spectral data using digital techniques are provided.